PARIS — France’s defense minister said Tuesday that French troops are involved in “very violent fighting” in the mountains of northern Mali and that it’s too early to talk about a quick pullout from the West African country, despite the growing cost of the intervention.

The fighting against Islamic extremists in the Adrar des Ifoghas mountains has been going on for days. A clash in the area killed 23 soldiers from neighboring Chad on Friday, according to a letter from French President Francois Hollande expressing condolences to his Chadian counterpart.

Soldiers from Chad and a few other African countries have joined the French-led operation to help Mali’s weak military push back extremists who had imposed harsh rule on northern Mali and started moving toward the capital last month.

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On Tuesday, the Obama administration imposed sanctions on an Islamic rebel leader whose extremist group seized much of northern Mali last year and prompted the French military intervention. The U.S. State Department designated Iyad Ag Ghali, head of the Islamic group Ansar Dine, a global terrorist. The action blocks any assets he holds in the U.S. and prohibits Americans from doing business with him.

The U.N. also added Ag Ghali to its global sanctions list.

Ag Ghali’s armed Islamic extremists conquered much of northern Mali after a military coup in Mali’s capital, aided by al-Qaida’s North Africa wing. In Timbuktu, he imposed strict Shariah law and forced thousands to flee; others were tortured and executed. But the French-led intervention in January has turned the tide, forcing back Ag Ghali’s rebels to mountainous hideouts near the Algeria border.

French Defense Minister Jean-Yves Le Drian said on France’s RTL radio Tuesday that the French intervention in Mali has cost more than (euro) 100 million ($133 million) since it started Jan. 11.

In the first weeks of the campaign, French and Malian forces easily took back cities in northern Mali. But the fighting is rougher now that it has reached more remote terrain in the mountains of the southern Sahara.

“We are now at the heart of the conflict,” in protracted fighting in the Adrar des Ifoghas mountains, Le Drian said. While some have suggested starting a pullout of the 4,000-strong French force next month, Le Drian said he couldn’t talk about a quick withdrawal while the mountain fighting goes on.

Hollande’s letter to Chadian President Idriss Deby said the deaths of Chadian soldiers “illustrate the dangers of this mission.” It gave no details. The Chadian army had initially said that 13 soldiers and 65 Islamic extremist rebels were killed in the fighting Friday.